1. Field of the Invention
The present invention relates to an RFQ linac which for instance, efficiently accelerates a charged particle beam having low energy, and particularly to an energy-variable RFQ linac preferably utilized in an ion implantation device or the like.
2. Discussion of Background
FIGS. 12 and 13 are respectively a sectional diagram and a partially broken side view of a conventional four-vane radio frequency quadrupole cavity which is for instance, described in the Proceedings of the 5th Symposium on Accelerator Science and Technology, p. 89-91, Sep. 26-28, 1984.
In FIGS. 12 and 13, a reference numeral 1 designates a cylindrical cavity which is a resonance cavity, and 2, a vane an end portion of which has a shape wavy in the longitudinal direction thereof. Four pieces of the vanes 2 are arranged on an inner wall face of the cylindrical cavity 1 so that the end portions of pairs of the vanes 2 oppose each other and the pairs are orthogonal to each other, which partition the cylindrical cavity 1 thereby forming four chambers 3. A numeral 4 designates a loop coupler provided at the cylindrical cavity 1, for supplying a high-frequency power into the cylindrical cavity 1, 5, a side tuner made of a cylindrical metal block which is provided at the inner wall face of the cylindrical cavity 1 facing the respective chamber 3 and 6, a driving device for the side tuner 5.
In this discussion, the cylindrical cavity 1 attached with the vanes 2 is called an acceleration cavity, all of which are composed of conductive bodies.
Next, explanation will be given to the operation of the conventional four-vane radio frequency quadrupole cavity (hereinafter, four-vane RFQ cavity).
When alternating voltages having the same polarity are applied to a first pair of the opposing vanes 2, and alternating voltages having the inverse polarity are applied to a second pair of the opposing vanes 2, a quadrupole electric field is generated in an aperture surrounded by the four vanes 2 for passing through the charged particle beam.
The charged particle beam receives a converging force of the quadrupole electric field and receives acceleration by an electric field generated in the proceeding direction of the charged particle beam owing to the wave shape of the end portions of the vanes 2.
The wave period of the end portion of the vane 2 is necessary to be a value which is proportional to a product of the wavelength of the alternating voltage by a beam speed, which is fabricated so that the period is prolonged with the acceleration of the beam.
Accordingly, once the vane 2 is fabricated, the speed of the beam emitted from the exit side of the cavity is determined by the frequency of the alternating voltage. Therefore, the only way to change energy of the beam emitted from the exit side of the cavity with respect to arbitrary charged particles, is changing the frequency of the alternating voltage.
The supply of the high-frequency power to the acceleration cavity is performed through the loop coupler 4. A magnetic field generated by the loop coupler 4 proceeds in the longitudinal direction of the acceleration cavity in the chamber 3 partitioned by the vanes 2, and towards the juxtaposed chambers 3 through a space at an end portion of the acceleration cavity, and returns. At this moment, a strong electric field is generated in a gap at the end portions of vanes 2. At the same time, an electric field is also generated at end portions of the opposing vanes 2 since electric charge is induced. As a result, the above alternating voltage is generated among the vanes 2.
To supply efficiently the high-frequency wave to the acceleration cavity, it is necessary to conform the frequency of the high-frequency wave to a resonance frequency of the acceleration cavity. A resonance frequency is determined by a product of a capacitance C by an inductance L both of which are connected in parallel in a general electric circuit, which is given by 2.pi.f.sub.r =(LC).sup.-1/2.
In this acceleration cavity, it is equivalently considered that an inductance L which is proportional to a sectional area of the chamber 3 and an intervane capacitance C are connected in parallel. Accordingly, when the frequency of the high-frequency wave is determined, it is necessary to determine a gap length between the vanes 2 and the sectional area of the chamber 3 so that the frequency agrees with a frequency of the cavity resonator.
Generally, the gap length between the vanes 2 is determined so that a high-density electric field is generated by a lowest possible voltage. Therefore, the sectional area of the chamber 3 is determined by the capacitance C which is determined by the above procedure, to obtain the necessary resonance frequency.
By extending the side tuner 5 into the chamber 3 and retracting it therefrom by the driving device 6, a space volume of the chamber 3 is changed thereby changing the inductance L. Therefore, the side tuner 5 is capable of changing the resonance frequency, and is used for adjustment of the resonance frequency and adjustment of an electromagnetic field distribution. In the above explanation, when the section of the cavity is not uniform in the longitudinal direction, the above factor should be considered as a space volume of the chamber 3. In this adjustment, the maximum changeable width of the resonance frequency is about 1%, which is an adjustment width for conforming the resonance frequency varied by a fabricating error of the acceleration cavity or the like, to a design value.
FIG. 14 is a longitudinal sectional diagram of a simplified structure of a conventional split coaxial RFQ cavity which is presented for instance, in the Symposium on Accelerator Science and Technology, 1989, FIG. 15, a sectional diagram taken along the line XV-XV of FIG. 14, and FIG. 16, a sectional diagram taken along the line XVI-XVI of FIG. 14. In these Figures, a numeral 7 designates an electrode the shape of the end portion of which is wavy in the longitudinal direction. In the cylindrical cavity 1, ends of a first pair of the electrodes 7 are electrically connected to an inner end face of the cylindrical cavity. The first pair of electrodes are opposingly arranged with respect to the axial line and extended in the axial direction, and shortcircuited to and supported by an inner wall face of the cylindrical cavity 1 by a stem 8b. Ends of a second pair of the electrodes 7 are electrically connected to the other inner end face of the cylindrical cavity 1. The second pair of the electrodes 7 oppose each other with respect to the axial line of the cylindrical cavity 1 and orthogonal to the first pair of the electrodes 7, extended in the axial direction of the cavity, and shortcircuited to and supported by the inner wall face of the cylindrical cavity by a stem 8a. Accordingly, this cylindrical cavity 1 is partitioned into three cavities by the stems 8a and 8b, and is equivalent to a structure wherein three split coaxial cavities are connected.
The cylindrical cavity 1 attached with electrodes 7 is called an acceleration cavity, all of which are constructed by conductive bodies.
Next, explanation will be given to the conventional split coaxial RFQ cavity.
When alternating voltages having the same polarity are applied to the first pair of the opposing electrodes 7, and alternating voltages having the inverse polarity are applied to the second pair of the electrodes 7, a quadrupole electric field is generated in an aperture surrounded by the four electrodes 7 for passing the beam. The beam receives a converging force by the quadrupole electric field, and also receives acceleration by a component of an electric field in the proceeding direction of the beam owing to the wavy shape of the end portions of the electrodes 7.
It is necessary to conform the period of the wave of the end portion of the electrode 7 to a value which is proportional to a product of the wavelength of the alternating voltage by the beam speed, and is fabricated so that the period is prolonged with the acceleration of the beam. Accordingly, once the electrode 7 is fabricated, the beam speed is determined by the frequency of the alternating voltage. Therefore, the energy of the beam emitted from the exit side of the cavity, is determined.
As a way of applying the alternating voltage to the electrode 7, a system is utilized wherein a high-frequency power is applied to the acceleration cavity, to rise a standing wave (resonant state). This system can efficiently supply power. Hereinafter, explanation will be given to why the above voltage is generated at the four electrodes 7 in the split coaxial cavity, and how the electromagnetic field is generated.
FIGS. 17(a) and 17(b) show a reentrant type cavity which is generally utilized for accelerating the charged particles. An outer conductive body 21 is equivalent to the cylindrical cavity 1, and an inner conductive body 22 is equivalent to the electrodes 7. In this cavity, the inner conductive body 22 disposed in the outer conductive body 21 of the cylindrical cavity, is separated at its center, by which an electric field is concentrated in a gap thereof, and the particles are accelerated by the electric field. The distribution of the electric field and the magnetic field and a path of a surface current are shown in the drawings. A potential difference between the separated portions of the inner conductive body 22 is uniform throughout the section of the cylinder of the inner conductive body 22.
FIGS. 18(a) and 18(b) show a modified example of the reentrant type cavity. In this cavity, a configuration for separating the inner conductive body 22 is changed thereby enlarging a domain capable of generating a strong electric field. Both the distribution of the electromagnetic field, and the path of the surface current are the same as those in the above reentrant type cavity, and the potential difference between the separated portions the inner conductive body 22 is uniform.
FIGS. 19(a) and 19(b) show a structure wherein one more pair of the separated portions of the inner conductive body 22 are added thereto. As shown in the electromagnetic field and the current path in the cavity structure combined with three cavities in FIG. 14, the inner conductive body 22 separated in two pairs, is equivalent to the electrodes 7. The voltages between the electrodes 7 become voltages necessary for generating the quadrupole electric field explained as above and are constant in the beam proceeding direction.
As stated above, the conventional split coaxial RFQ shown in FIG. 14 is equivalent to the structure wherein the three split coaxial cavities shown in FIG. 19 are connected. The connecting portions have structures shown in FIGS. 15 and 16, because a single vacuum pump will do for maintaining the inside of the cavity in vacuum, and, when a high-frequency power is supplied from one position, the high-frequency wave is easily propagated throughout the cavity. Even without these connecting portions, the same voltage distribution as in the above split structure can be obtained. However, since the electrode 7 is supported at one position, when it is elongated, it becomes mechanically unstable and not practical. Therefore, generally, to stabilize the electrode, a reinforcement 9 is attached to the electrode 7 as shown in FIGS. 20(a) and 20(b).
To efficiently supply the high-frequency power to the acceleration cavity, it is necessary that the frequency of the high-frequency wave agrees with the resonance frequency of the acceleration cavity. In a general electric circuit, the resonance frequency is determined by a product of a capacitance C by an inductance L which is parallelly connected thereto, as 2.pi.f.sub.r =(LC).sup.-1/2.
In this acceleration cavity, the capacitance C is given as a sum of an intervane capacitance C.sub.VV and a vane-stem capacitance C.sub.VS between the electrode 7 and the stem 8. The inductance L is obtained from a tank inductance L.sub.T obtained by a magnetic field surrounding the electrode 7 and L.sub.s obtained by a magnetic field surrounding the stem 8 by the following equations. EQU L=(L.sub.T /3).(L.sub.T +3L.sub.S)/{L.sub.T +(L.sub.S /3)} EQU L.sub.T =(.mu.0/2.pi.).1.sub.m.1n(r.sub.E)
where l.sub.m is a length of the electrode partitioned by the stem 8, r.sub.C, an inner radius of the cylindrical cavity 1, and r.sub.E, an effective radius of the electrode 7. Accordingly, when the frequency of the high-frequency wave is determined, a gap length between the electrodes 7 and a sectional area of the cylindrical cavity 1 should be determined so that it agrees with the frequency of the acceleration cavity. Generally, the gap between the electrodes is determined so that a high electric field is generated by the lowest possible voltage. Therefore, the sectional area of the chamber is determined by the capacitance C determined by the above procedure to obtain the necessary resonance frequency.
However, when the cavity is actually fabricated, the resonance frequency is slightly deviated since there always is a fabricating error. To correct the error, generally, the side tuner 5 made of a metal block is attached to the cavity as in the conventional four-vane RFQ shown in FIG. 12, by which the resonance frequency is finely controlled by equivalently changing L.sub.T by pushing in and pulling out the side tuner 5. In this adjustment method, the maximum changeable width of the resonance frequency is about 1%.
As stated above, in the conventional four-vane RFQ and the conventional split coaxial RFQ, the speed, or the energy of the beam emitted from the exit side of the cavity can not considerably be changed. Therefore, they are used as a primary stage of a high energy accelerator utilized in an atomic nucleus experiment wherein the energy is not necessary to be varied.
Since the conventional four-vane RFQ and the conventional split coaxial RFQ are constructed as above, the speed or the energy of the beam emitted from the cavity can not considerably be changed. Therefore, these devices are not applicable to an ion implantation device wherein the energy is required to be considerably variable with respect to the same charged particle.